The following is a list of publications disclosing methods and apparatus for beam steering and for mixing and coupling of laser beams in photorefractive media:
(1) "Electro-Optic and Acousto-Optic Scanning and Deflection", M. Gottlieb, C. L. M. Ireland and J. M. Ley, Optical Engineering, Vol. 3 (1983), Marcel Dekker, Inc., New York and Basel;
(2) "Laser Scanning and Recording: Developments and Trends", Leo Beiser, Laser Focus/Electro-Optics, Feb. 1985 pps 88-96;
(3) "Opto-Optical Light Deflection", G. T. Sincerbox and G. Roosen, Appl. Optics 22, 690 (1983);
(4) "Dynamic Beam Deflection Usxing Photorefractive Gratings in BSO Crystals", G. Pauliat, J. P. Herriau, G. Roosen and J. P. Huignard, J. Opt. Soc. Am. B3, 306 (1986);
(5) U.S. Pat. No. 4,869,579;
(6) "Self Bragg matched beam steering using the double-color pumped photorefractive oscillator", Appl. Phys. Lett. 51, 74 (1987);
(7) "Photorefractive Oscillators", B. Fischer, S. Sternklar, S. Weiss, IEEE J. Quant. Electron 25, 550 (1989);
(8) "Beam Coupling and Locking of Lasers using Photorefractive 4 Wave Mixing", S. Sternklar, S. Weiss, M. Segev and B. Fischer, Opt. Lett. 11, 528 (1986), made the subject of commonly-owned Israel patent application No. 79581, filed Jul. 31, 1986 and incorporated herein by reference;
(9) "Phase-conjugate Nd: YAG laser with internal acousto-optic beam steering", J. L. Aycol, J. Montel, T. Verny and J. P. Huignard, Opt. Lett. 16, 1225 (1991); and
(10) "High-power beam steering using phase conjugation through Brillouin-induced four-wave mixing", D.C. Jones, G. Cook, K. D. Ridley and A.M. Scott, Opt. Lett. 16, 1551 (1991).
The prior art includes mechanical (mirror scanning), acousto-optic and electro-optic methods and apparatus for beam steering.
Mechanical methods have the disadvantages of being bulky, slow and relatively inaccurate (e.g. susceptible to vibrations and flutter). Acousto-optic crystals are widely used. However their maximum steering angle is about .+-.2.degree., limited by the Bragg Condition. In addition, their use is limited to low power continuous wave (CW) beams due to the trade-off between access time and beam diameter in these devices, which dictates the use of focused beams in the acousto-optic crystal. Due to use of focused beams they must have low power in order to prevent damage. This imposes a trade-off between the deflection resolution of these devices and their access time.
Refs. 3 and 4 disclose a complex technique based upon photorefractive wave mixing for overcoming the Bragg condition. This technique requires a relatively large amount of wavelength tuning, of the order of 5%, to achieve angular steering of about 12.degree..
U.S. Pat. No. 4,869,579 and related Refs. 6-8 disclose an improved system and method of steering two pumping beams with demonstrated steering angles of about .+-.3.degree., based on automatic Bragg matching and in which the input pumping beams may have different wavelengths. In accordance therewith the two incident pumping beams impinge on two sides of a third order, non-linear photorefractive polarization medium and have a predetermined alignment and spatial overlap in the interaction region of the polarization medium at which they are coupled and self-diffracted. Each of the two incident pumping beams generates a self-diffracted mate and defines together therewith a beam couple. The beam couples produced from the two incident pumping beams are operative to write a common grating in the photorefractive polarization medium such that there emerge from the medium two steered output beams with controllable offset angles, and optionally with spatial and temporal modulation.
Controlling the steering angle .THETA. in accordance with U.S. Pat. No. 4,869,579 is based on changing the wavelength .lambda. of one pumping beam relative to the other. In case .lambda. of the two pumping beams is a priori the same, the steering angle .THETA. within the non-linear medium obeys the formula: ##EQU1## where .psi. is the angle between the two input pumping beams within the non-linear medium to be referred to hereinafter as "convergence angle", and d.lambda. is the difference in wavelength .lambda. between the two input pumping beams resulting from wavelength modulation. The above formula is an approximation which assumes exclusively obtuse convergence angles of up to nearly 180.degree.; and ##EQU2## so that .THETA.&lt;1 Rad.
Generally, the convergence and output angles outside the medium will be different than inside the medium due to the Fresnel refraction at the medium/ambient interface with the exception of those cases in which the Fresnel refraction valve is 1. Hereinafter, the terms convergence and output angles are meant to refer to the occurrence within the medium.
Although the system and method of U.S. Pat. No. 4,869,579 and other photorefractive methods overcomes some drawbacks of the state of the art at that time, it still has several intrinsic limitations. For one, although U.S. Pat. No. 4,869,579 mentions that steering occurs in consequence of variations of the wavenumber k the only available parameter for control is the wavelength .lambda., and as relatively large variations d.lambda. are required due to the small value of sin.psi. (d.lambda./.lambda..apprxeq.0.1), the steering is not sufficiently flexible. Therefore, the very nature of the system and method according to the U.S. patent implies that to achieve large enough d.lambda.the steering is achieved either by bulky and slow mechanical means or by sophisticated acousto-optical or electro-optic means, which latter are expensive and not readily available and increase radically the cost of the entire system to an extent which, from a commercial point of view, may be prohibitive.
Still further, the system and method of U.S. Pat. No. 4,869,579 are unsuitable for the steering of high intensity beams and in consequence many military and civilian applications are excluded.
It is the object of the present invention to provide improved system and method for steering and deflecting laser beams that will overcome the above drawbacks.